1. Field of the Invention
The present invention relates to an automatic transmission for a chain-driven rotating wheel. In particular, the invention relates to a linked chain-driven wheel where the ratio of the linear speed of the chain to the angular speed of the wheel about the hub thereof is automatically and substantially continuously adjusted. The invention finds particular application in the provision of an automatic transmission for a bicycle and like-pedaled apparatus.
2. Background of the Invention
In recent years, bicycling has enjoyed a great revival of interest. This revival has been due to an increased emphasis on exercise and physical well-being, as well as environmental considerations such as pollution problems from internal combustion exhausts and economic considerations such as the increasing expense of alternate means of transportation. This has caused bicycles to be used in a variety of riding conditions, in towns and cities and in the country, and for recreation or commutation. Due to the wide variability of bicycle uses, multi-speed bicycles, particularly 10speed bicycles, have come into wide-spread use. These bicycles provide means for varying the transmission ratio between the pedaled sprocket and the rear wheel of the bicycle, the ratio being adjusted in discrete increments by the rider in accordance with speed and/or grade of terrain. The conventional transmission changing mechanism on 5- and 10-speed bicycles is the so-called "derailleur" mechanism. The "derailleur" utilizes multiple side-by-side sprockets of different diameters and rider- controlled levers to shift the bicycle drive chain from one sprocket to another of greater or lesser diameter.
The "derailleur" is subject to a number of well-known problems. The chain jumping from sprocket to sprocket can "hang up". The lateral displacement of the chain between its front and rear sprockets, in certain gears, increases the frictional force exerted on the chain and sprockets, making pedaling difficult and increasing wear on the parts of the mechanism. Moreover, shifting is limited to discrete incremental changes in ratio since all of the sprockets are of fixed diameter.
Many attempts have been made to provide automatic and/or continuously variable transmissions for bicycles.
Hayot, U.S. Pat. No. 2,584,447, discloses a variable-speed chain driving device comprising a hub mounted on the pedal shaft of a bicycle, the hub having mounted thereon flexible arms which carry at their extremities sprocket rollers which drive the chain of the bicycle. With increasing tension of the chain, due to increasing resistance offered by the rear wheel, e.g. cycling uphill, the flexible arms tend to wrap around the hub to decrease the effective diameter of the drive sprocket defined by the sprockets mounted on the arms, thereby reducing the transmission ratio to a fixed diameter sprocket driving the rear wheel. Thus, the transmission ratio is varied continuously and automatically in response to varying rear wheel resistance.
Hunt, U.S. Pat. No. 3,798,989, discloses a manually-adjustable, multiple-speed variable transmission for a bicycle wherein the front and/or rear drive sprockets can be incrementally varied in diameter by the operator.
Chao, U.S. Pat. No. 3,837,234, discloses a bicycle with a manually-operated stepless transmission wherein the rider can manually change the depth of groove between a pair of V sheaves, which define a V-belt pulley, the V-belt transmitting force to the rear wheel of the bicycle.
Tompkins, U.S. Pat. No. 3,956,944, discloses a torque operated, variable diameter, belt sheave or chain sprocket wherein a variable diameter drive wheel is formed by sheave or sprocket elements carried between two pairs of discs. If one pair of discs rotates with respect to the other pair, as a result of a change in torque force, the sheave or sprocket elements move radially inward or outward to vary the diameter of the drive wheel. Additionally, this patent discloses a free-wheel mechanism which allows a rider to "coast" on the bicycle. In particular, a plurality of individual teeth define the diameter of a drive sprocket for a bicycle. Each of these teeth is carried by a carrier, in which the tooth is slidable. If the tooth is contacted by the bicycle chain on its rearward surface, the tooth will engage the chain and transmit drive pressure. However, if the tooth is contacted by the chain on its forward or upper surface, the tooth will be depressed into the carrier for the tooth and allow the chain to slide over the tooth.
Pipenhagen, Jr., U.S. Pat. No. 3,969,948, discloses an automatically variable speed ratio transmission for a bicycle, comprising a pedal-operated drive sprocket of variable diameter formed from a plurality of small sprockets mounted on bell-crank arms. The diameter of the so-formed sprocket being variable with the resistance to pedaling.
Leonard, U.S. Pat. No. 4,030,373, discloses a variable speed drive for a bicycle comprising a variable diameter drive mechanism formed by a mounting plate having a number of radial tracks therein, and a number of sheave elements are each movably mounted in the tracks. The rider can manually cause the elements to move in the tracks to vary the diameter of the sheave pulley formed by the elements.
Porter, U.S. Pat. No. 4,119,326, discloses a variable speed bicycle wherein the bicycle is driven by a set of variable diameter pulleys. In particular, each pulley is a V-belt pulley formed of two halves, the spacing between the halves defining the diameter at which a V-belt of fixed width will ride in the pulley. The spacing of the pulley halves is manually adjusted by the rider.
Williams, U.S. Pat. No. 4,342,559, discloses a drive system useful for a bicycle comprising a load-responsive variable diameter pulley. The variable diameter pulley is formed of a fixed plate and a movable plate, each plate is provided with slots and belt engaging elements ride between the plates and engage respective pairs of slots. As the load on the drive belt varies, the plates rotate with respect to each other and cause radial movement of the belt engaging elements, thereby varying the diameter of the pulley.
In addition to load-operated and manually adjustable transmissions, attempts have been made to produce transmissions responsive to speed of rotation.
Hohne, U.S. Pat. No. 904,330, discloses a driven shaft having a hub mounted thereon. This hub comprises a number of fixed spokes. The spokes have U-shaped crosssections and in each fixed spoke there is slidably mounted a sliding spoke which is formed at its outer end as a sector of the periphery of a belt pulley. Springs are provided which tend to pull the sectors to maximum diameter. The sliding spoke carries a pin which is engaged by a fork. This fork is pivoted about a point and carries a weight at its end. The operation of the apparatus is such that if the speed of rotation of the shaft increases, the weights will tend to move outwards and thereby exert an inwardly directed pull on the sectors. The inward movement of the sectors diminishes the diameter of the pulley. Likewise, when the speed of rotation of the shaft decreases, the weights will move inwards, aided by the springs, and thereby increase the diameter of the pulley.
Lee, U.S. Pat. No. 3,935,751, discloses a centrifugal, stepless speed changing device comprising a plurality of claw blocks mounted pivotally on a rotating disc. The outer ends of the claw blocks defining the periphery of a drive wheel. As the speed of rotation of the disc increases, the claw blocks move outward to increase the diameter of the wheel. Likewise, when the speed of rotation of the disc decreases, the claw blocks move inward to decrease the diameter of the wheel.
Also, many attempts have been made to produce variable diameter pulleys. Typically, such variable diameter pulleys are V-belt pulleys comprised of a pair of opposed cone discs or pulley halves which cooperate with a V-belt, the variation in diameter being achieved by adjusting the axial spacing of the pulley halves which allows the V-belt to ride higher or lower in the groove between the pulley halves. Representative of such variable diameter V-belt pulleys are Tyler, U.S. Pat. No. 2,321,438; Carroll, U.S. Pat. No. 2,491,248 (disclosing a remote control linkage and idling mechanism); Ingold, U.S. Pat. No. 2,751,790 (disclosing a mechanism for simultaneous, equal and opposed movement of the pulley halves to prevent lateral movement of the belt); Wrobbel, U.S. Pat. No. 2,941,412 (a further mechanism for simultaneous, equal and opposed movement of the pulley halves); Aplin, U.S. Pat. No. 3,064,486 (a still further mechanism to assure belt alignment by achieving simultaneous, equal and opposite movement of the pulley halves); Schaufler, U.S. Pat. No. 3,115,045 (a still further mechanism to assure belt alignment with diameter variation) and Looker, U.S. Pat. No. 3,269,201 (a mechanism designed to prevent intrusion of dirt into and to preclude loss of lubricant from the pulley).
As may be readily ascertained from the abovenoted patents, the prior art is replete with attempts to provide variable speed transmissions for bicycles by utilizing pulleys (drive wheels or sprockets) of variable diameter and, more generally, to provide pulleys of variable effective diameter. These attempts suffer from numerous defects. With respect to V-belt pulleys of variable effective diameter, complicated mechanisms are necessary to ensure that the V-belt is not laterally displaced when the pulley halves are moved relative to one another to vary the effective diameter of the pulley. These mechanisms increase the cost of the pulley and make manufacture of the pulley more difficult. Without such a mechanism, the lateral displacement of the V-belt causes uneven and rapid wear of the belt during operation.
Moreover, belt-type drives for bicycles have generally been found to be inefficient and troublesome. In particular, belt drives tend to slip over the pulley when heavily loaded as when starting up on a bicycle or when traveling over a steep grade in the terrain. This slippage tendency is exacerbated in wet road conditions where water acts as a lubricant between the belt and pulley. Additionally, road dirt tends to act as an abrasive and increases the wear on the belt.
Mechanisms which solely rely on the speed of rotation of a driven shaft to vary drive wheel diameter, such as Hohne and Lee, mentioned above, fail to take into account the variability of the driven load. Thus, while a given load will allow operation of these devices as indicated, an increase in load will increase the belt tension thereby increasing the force exerted on the variable diameter wheel. This increase in force on the wheel will tend to cause it to decrease in diameter, thereby disadvantageously affecting the speed ratio. Additionally, the mechanisms of Hohne and Lee are directed to belt drives which are susceptible to the previously noted disadvantages, per se.
Attempts to adapt variable diameter wheels to bicycles, as illustrated by the above-noted patents, have been limited to torque or load-operated mechanisms or to manually adjustable systems.
A need, therefore, continues to exist for an automatic speed ratio transmission for bicycles and like-pedaled apparatus.